Answer:
Active.
Explanation:
First one must recognize the difference between active versus passive voice. Active voice is where the subject is put before the predicate, and passive is where the predicate ie put before the subject.
Example:
Sally <em>drove her car. </em>--> Active voice, because subject is first.
Subject <em> </em><em>predicate</em>
<em>----------------------------------------------------------------------------------------------</em>
<em>The car was driven </em>by Sally --> Passive voice, because predicate is first.
<em> predicate </em> Subject
<em>----------------------------------------------------------------------------------------------</em>
Furthermore, you can break down the predicate into the object and action. Using the same example below...
Sally <em>drove </em><u>her car </u> --> Active, because subject is first.
subject <em>action </em>object<em> </em>
<em>----------------------------------------------------------------------------------------------</em>
<u>The car </u><em>was driven by </em>Sally--> Passive, because action and object are first.
<u>Object</u><em> action</em> Subject
<em>----------------------------------------------------------------------------------------------</em>
In your example, your subject (Spectators) are first and your action and <em>object</em> (filled <em>the</em> <em>stadium</em>) are second. Therefore, it is active voice. (also, remember that your action and object can be combined to form the <u>predicate</u>.)
Hope this helps!